The Ivorian cabinet has resigned over the incident, and on Friday angry crowds burned the house of the port director. But some dumps are still reported to be unfenced, with children playing in them.

Treatment of the victims will depend on what poisoned them, which remains a mystery. Experts from the UN Environment Programme have flown to Abidjan to investigate. “Foreign experts” were reported as saying in Abidjan on Friday that the waste contains hydrogen sulphide (H2S), while the company that owns the waste says that would be “amazing”.

But hydrogen sulphide has a characteristic smell of rotten eggs, and such a smell is said to have been hanging over Abidjan since the dumping. In fact, the stench seems to be why the mess arrived in Abidjan in the first place.

Unloading slops

On 2 July 2006 the tanker Probo Koala arrived in Amsterdam from Gibraltar, and asked Amsterdam Port Services (APS) to unload its “slops” – residues from washing cargo tanks with caustic soda. The ship was on contract to Trafigura Beheer, a Dutch commodities trader, to carry petroleum derivatives used to make gasoline.

Such slops are normally handled under the 1978 Marpol treaty on pollution from ships. “But when they pumped the stuff out, it smelled of rotten eggs,” says Jim Puckett of the Basel Action Network in Seattle, US, a pressure group that monitors the toxic waste trade.

That meant it was not regular Marpol waste, and had to be handled differently. APS demanded more money. “The ship’s operators decided the price was not justified,” a spokesperson for Trafigura told New Scientist.

They pumped the waste back on board and sailed off, stopping in Estonia and Nigeria and eventually contracting with Compagnie Tommy of Abidjan to take the waste. Trafigura denies reports that Tommy is owned by Puma Energy, a Trafigura subsidiary.

APS had the waste chemically analysed “but they did not test for hydrogen sulphide”, says a spokesperson for Trafigura. “I can’t say there couldn’t be hydrogen sulphide there, but I would be amazed.”

Breathing difficulties

The effects of low-level exposure to hydrogen sulphide can be delayed, and include headache, dizziness, weakness, sore throat, nausea, nosebleeds and breathing difficulties. These symptoms have been reported by people in Abidjan.

Michael Costigan, of the UK's Health and Safety Executive says exposure to “a few hundred parts per million” of the gas induces eye irritation and breathing difficulties, while 1000 parts per million for even a few minutes – or half that level for a few hours – causes unconsciousness and death.

Hydrogen sulphide is a metabolic poison, like cyanide, and regularly kills people who work in sewers, the oil industry and other affected workplaces.